A variety of digital watermarking techniques have been proposed to embed copyright information in digitalized copies of images and music. Digitalized data is readily copied in a complete form (namely, digitalized data has perfect reproducibility), so that the protecting measure is required against illegal copies. The digital watermarking technique electronically embeds watermark information, such as copyright information, in master data in a human-imperceptible manner. The embedded watermark information can be taken out according to the requirements. The digital watermark clearly shows any third party the presence of the copyright in digitalized copies of images and music. The digital watermark generally includes information for identifying the copyright holder. The data with the digital watermark embedded therein may thus be referred to as the authorized data.
The prior art digital watermarking technique, however, has the drawback that the legal watermark information is not taken out accurately when another piece of information overwrites the existing information by a similar procedure. Alteration to invalidate the digital watermark embedded in the authorized data may hereinafter be referred to as the overwriting attack against the digital watermark. When the authorized data is exposed to the overwriting attack to prevent the embedded legal digital watermark from being normally read, the digital watermarking loses its significance. If only an unauthorized watermark is left as the result of the overwriting attack, this allows even the illegal alteration of the signature of the copyright holder.
Electronic data are generally compressed for the purpose of delivery or storage. The digital watermark data embedded by the prior art digital watermarking technique, however, may be altered or even deleted by such data compression. Namely the prior art technique is not sufficiently practical.